1. Field of the Invention
This invention relates to an adhesive composition for fibrous materials, in particular, polyester fibers and more particularly, it is concerned with a novel adhesive composition capable of bonding strongly polyester fibers and rubbers with reduced adhesive deterioration even at high temperatures.
2. Description of the Prior Art
Polyester materials, linear macromolecules having ester bonds in the main chains typical of polyethylene terephthalate are very useful as reinforcing materials of rubber articles such as tyres, belts, air springs and rubber hoses in the form of filaments, yarns, codes, cables, code textiles and ducks, because of having more stable dynamic properties, lower stress relaxation, lower creep property and better recovery after stretching than other materials such as nylons.
When using polyester fibers as reinforcing materials of these rubber articles, however, bonding between the rubber and reinforcement is hardly obtained with RFL adhesive consisting of a resorcinol/formaldehyde initial condensate and rubber latex capable of well bonding nylons or rayons and rubbers, since the polyester fibers are dense in highly dimensional structure and have few functional groups, and thus the polyester fibers are not suitable for the reinforcing materials of rubber articles. Therefore, a number of adhesive compositions and methods have been proposed, for example, a method comprising treating a polyester surface with an alkali or amine to increase active functional groups on the surface such as --OH, --COOH and --NR.sub.2 wherein R is hydrogen or an alkyl group or a method comprising incorporating a compound having groups capable of bonding with hydrogen or making primary bonding by using isocyanate compounds or epoxy compounds onto a polyester surface and then treating with RFL adhesives.
However, the treatment with an alkali or amine has such as disadvantage as deteriorating the tensile strength of polyester fibers, and the isocyanate compounds or epoxy compounds are highly reactive and thus react with water, as a solvent, in RFL and RF in the RFL components, resulting in large difficulty in preparation of a one liquid composition adhesive and in deterioration of the bonding property. In this case, therefore, the process of treating polyester fibers with the adhesives must be carried out separately in two stages, which needs additional installations, steps and heat to that extent, and this is not preferred from the standpoint of saving sources and energy. Furthermore, the isocyanate compounds or epoxy compounds are not preferable in view of bad influences upon the human body due to toxic fume or vapor occurring when treated in the air or when subjected to a heat treatment and in view of the environmental pollution caused thereby.
In performances, polyester fibers or the surfaces thereof tend to be hardned, thus resulting in difficulty of handling, and the most serious disadvantage of polyester fibers is that when using the polyester fibers subjected to the adhesive treatment as described above under a high strain or at a high temperature, there takes place a rapid deterioration of the adhesion and the tensile strength of the fibers, and the life of rubber articles is markedly shortened. The reason therefor is that since an isocyanate compound or epoxy compound is primarily bonded with a polyester fiber on the surface thereof to form a kind of graft polymer (skin-core structure), the polyester molecule chain is decomposed by hydrolysis of the polyester at a high temperature or by aminolysis due to the residual amines of a vulcanization accelerator in a compounded rubber, thus resulting in a bonding breakage on this part in easy manner, and the polyester fiber becomes mechanically brittle under a highly strained state due to a difference in rigidity between the surface and inside of the polyester fiber.
Of late, tyres have been directed to a tendency of light weight from the standpoint of saving energy and it has increasingly been required to improve the stability of a tyre against thermal or mechanical inputs and lengthen the life thereof until breakage. Thus, the bonding force between polyester fibers and rubbers must sufficiently be stable and tenacious against such inputs more than required for the life of a product.
As adhesives for polyester fibers in the form of one liquid composition, there have been proposed a number of adhesive compositions as exemplified by reaction products of resorcinol, p-chlorophenol and formaldehyde (U.S. Pat. No. 3,660,202 and Japanese Patent Publication No. 11,251/1971) and reaction products of resorcinol and triallyl cyanurate. However, these adhesives are not satisfactory, since the bonding force is insufficient and there occurs deterioration of the bonding and the strength of fibers during continuous use at a high temperature and a high strain.
As described above, it is eagerly desired to develop an ideal adhesives for polyester fibers, which can be used in the form of one liquid composition and can realize a tenacious bond to the polyester by one stage processing even at a high temperature, and which can be freed from lowering of the strength of polyester fibers during processing and using, environmental pollution due to occurrence of toxic fume or vapor and deterioration of the bonding during continuous use at a high temperature and strain.